1. Field of the Invention
This invention relates to a novel liquid crystalline carbonate and liquid crystal medium containing the same with positive or negative dielectric anisotropy. The medium is intended for applications in various photonic devices and displays, especially as light beam modulators working in the different ranges of electromagnetic spectrum.
2. Background of the Invention
Nematic liquid crystalline media with positive or negative dielectric anisotropy are commonly applied in displays and photonic devices (optical valves, phase shifters, attenuators etc.) which use various electrooptical effects. One of the most widely known is ECB (Electrically Controlled Birefringence), described for the first time in articles [M. Schiekel, K. Fahrenschon, Appl. Phys. Lett, 19, 391 (1973); G. Labrunie, J. Robert, J. Appl. Phys., 44, 4869 (1973) and S. Matsumoto, M. Kawamoto, K. Mizunoya, J. Appl. Phys., 47, 3842 (1976)]. Both Δ∈>0 and Δ∈<0 liquid crystal media can be applied for this purpose.
In the mentioned effect a liquid crystalline medium (a mutlicomponent nematic mixture, preferably of eutectic composition) characterized by negative or positive dielectric anisotropy is placed between two transparent electrodes, or one transparent and one reflective electrodes.
In the former case a liquid crystal with negative dielectric anisotropy (Δ∈<0) is aligned perpendicularly to the electrodes surface in the off state (without electric field). When external electric field is applied the reorientation of liquid crystalline molecules occurs to parallel (planar) position in respect to the electrodes surface. This mod is also called Vertical Alignment and has been modified many times (MVA, PVA, ASV) in order to improve the viewing angle.
In the latter case when a liquid crystal of positive anisotropy (Δ∈>0) and homogenic alignment (parallel to electrodes surfaces) is applied, then in the presence of external electric field its molecules are reoriented perpendicularly to the electrode surface.
The fitting condition of the optical path to the thickness of the cell filled with liquid crystal is given by an equation:d·Δn=λr/2where: d—liquid crystal cell thickness, Δn—birefringence of liquid crystal, λr—wavelength of light beam passing through the liquid crystal. From this dependence, it can be concluded that the devices of smaller thickness demand the liquid crystal medium of higher birefringence. At the same time, because of the Δn decreases with increasing the wavelength, for devices operating at infrared region (for telecommunication applications λr˜1.5 μm is preferred) high birefringence liquid crystals are needed. For this wavelength region the necessary value of birefringence Δn is 0.15 for d=5 μm and 0.25 for d=3 μm. It corresponds to values of Δn in the range of 0.25-0.3 for the first example and 0.45-0.5 for the second example in visible light (λ=600 nm).
Threshold voltage for reorientation in ECB effect is determined by an equation:
      V    th    =                              π          2                ⁢                  K          33                                      ɛ          o                ⁢                                        Δ            ⁢                                                  ⁢            ɛ                                        
When the device is controlled by an active matrix, Δ∈ cannot be too large, because of undesired increase of the conductivity. Optimum Δ∈ values are between 3 and 9 and they assure the controlling voltages at the 5 to 7 V range.
Response times are given by equations:
            t      on        ∝                  γ        ·                  d          2                            Δ        ⁢                                  ⁢                  ɛ          ⁡                      (                          V              /                              V                th                                      )                                ,            t      off        ∝                  γ        ·                  d          2                            k        ii            where reorientation times (ton rise time and toff fall time) depend on the material constants of the used liquid crystal (rotational viscosity γ, dielectric anisotropy Δ∈, elastic constants kii—various depending on the effect) as well as on the cell geometry and its driving parameters (thickness d and applied voltage V to threshold voltage Vth).
The decrease of the liquid crystal layer thickness leads to the decrease of response times. An important parameter is the temperature dependence of optical response times especially in the case of high power laser applications.
Most of the currently used liquid crystal media possess the birefringence from 0.1 to 0.14. They are usually cyclohexyl, bicyclohexyl and phenylocyclohexyl derivates of benzene substituted with one, two or three fluorine atoms, see [S. M. Kelly, M. O'Niel Handbook of Advanced Electronic and Photonic Materials and Devices, edited by H. S. Nalwa, Vol 7: Liquid Crystals, Displays and Laser Materials (2000), R. Dqbrowski, Liquid crystalline materials for active matrix displays, Biul. WAT, 48, No. 4, p. 5 (1999)].
In order to obtain liquid crystal mixtures of higher birefringence biphenyl, terphenyl, tolane, phenyl-tolane and even quaterphenyl and/or biphenyltolane derivates can be applied. Many of such compounds were described in patents [EP 0 704 512, EP 0739 876, DE 42 03 719, EP 0 733 692, EP 0 761 799, EP 89 10 3414, WO 97/38062, DE 43 26 020, DE 38 07 958, DE 42 22 371, EP 0 575 791, EP 0 736 513, EP 0785 179], and in review articles. [R. Dqbrowski, Liquid crystalline materials for active matrix displays, Biul. WAT, 48, No 4, p. 5 (1999), M. Hird, Fluorinated liquid crystals—properties and applications, Chem. Soc. Rev., 36, 2070-2095 (2007), P. Kula, A. Spadlo, J. Dziaduszek, M. Filipowicz, R. Dqbrowski, J. Czub and S. Urban, Mesomorphic, dielectric and optical properties of fluorosubstituted biphenyls, terphenyls and quaterphenyls, Opto-Electronics Review, 16(4), 379-385 (2008)].
The length of alkyl and alkyloxy terminal chains, the positions of the lateral substituents and the character of a terminal polar group are crucial factors for observed mesogenic properties: melting points, clearing points, the type of liquid crystal phases and their transitions temperatures, viscosity, dielectric anisotropy, optical anisotropy and elastic constants. Although their relation to chemical structure is better known nowadays, there are still many unpredictable and surprising correlations, which have influence on devices performance, therefore further experimental research is needed.
Fluorine atoms substituted in lateral positions often decrease melting points and depress smectogenity in a favorable way. At the same time their presence leads to the decrease of clearing point and birefringence. The optimum number of fluorine atoms is from 1 to 3.
The esters of carbonic acid prepared from cholesterol [W. Elser, W. J. L. Pohlmann, P. R. Boyd, Mol. Cryst. Liq. Cryst., 20, 77 (1973)] or other alcohols or hydroxy esters [D. Demus, Flussige Kristalle in Tabellen, VEB DeutscherVerlagfur Grunstqffindustrie edition, Leipzig 1974, p. 73] are known.
Liquid crystal compounds with terphenyl rigid core and carbonate group in a terminal chain (H2n+1CnOCOO or H2n−1CnOCOO) have not been obtained or investigated yet (in Liquid Crystals database there are no three ring compounds with H2n+1CnOCOO or H2n−1CnOCOO terminal groups) but in [U.S. Pat. No. 4,594,465] patent besides alkyl and alkyloxy terphenyl derivates also carbonates with formulas 1, 2 and 3:
wherein R1 or R2 are alkyl groups H2n+1Cn have been claimed, although their mesogenic properties were not presented in any form.